Nitrogen liquefaction with plural work expansion of feed as refrigerant

ABSTRACT

1,099,669. Gas liquefaction process. AIR REDUCTION CO. Oct. 7, 1966 [Oct. 8, 1965], No. 44923/66. Heading F4P. Liquefaction of a gas, e.g. effluent nitrogen from the low pressure column (103), Fig. 6 (not shown), of a two stage air rectification plant is effected by compression to not more than 2À5 times the critical pressure of the gas at 151, 153, 155, 157, 159, 163, 166 cooling in means including paths a, a&lt;SP&gt;1&lt;/SP&gt; of a first heat exchanger 169, dividing the cooled gas at 186 into a minor portion which is cooled without liquefaction in path a of a second exchanger 192 and into a major portion which is work-expanded in turbines 188, 194, with intervening warming in a path a of exchanger 192 from which latter the cooled minor portion is passed through a line 203 and is throttle-expanded at 204, 209, 213 into a product liquid storage vessel 213; uncondensed vapour from the latter being forced by a pump 217 through paths c of exchangers 192, 169 and a line 221 to the compressor inlets where it is joined by the work expanded major portion. The compressed feed gas is withdrawn from path a of exchanger 169 cooled by external refrigeration means 175 and is then returned to path a of exchanger 169. Throttle valve 204 discharges into a separater 206 which also condenses oxygen. Vapour above liquid oxygen storage vessel 112 and a portion of condensed nitrogen is passed along a line 122 to provide additional reflux for the column (103).

Dec. 19, 1967 D. L. SMITH ETAL 3,353,460

NITROGEN LIQUEFACTION WITH PLURAL WORK EXPANSION OF FEED AS REFRIGERANTFiled Oct. 8, 1965 s Sheets-Sheet 1 I so FIG.

' TEMPE/M TURE-ENTHALPY PLOT FOR COLD EXCHANGER x I60 I80ATM NITROGEN ICOOLING CURVE Lu I40 it b Y L; I30 Q: l20 E I I HEA TING cum/.1:

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l l l I l l l 240 200 I60 l so o ENTHALPY CHANGE TEMPERA TURE-ENTHALPVPLOT l 50 FOR COLD EXCHANGER 45 A TM NITROGEN a v COOL ING CURVE u Qzl bso 2 E12o H g1 10 u loo HEATING cum/E 2 .0 260 '30 |'2o 8'0 4'0 0ENTHALPV CHANGE DONALD L. SMITH B ATTORNEY Dec. 19, 1967 D. L. SMITHETAL 3,353,460

NITRQGEN LIQUEFACTION WITH PLURAL WORK EXPANSION OF FEED AS REFRIGERANTFiled Oct. 8, 1965 e Sheets-Sheet 2 7 FIG. 3

V TEMPERATURE-ENTHALPY PLOT FOR COLD EXCHANGER 45 ATM NITROGEN COOL INGCURVE ac Q5 E l 130- I E REHEATSECT/ON q Q 7 i' 120 I 3 8 6c LIL! RE LA7'/ l/E POSITIONS OF ||O TURB/NES IN cow END E CYCLE V I HEAT/N6 CURVE iIOO- l I v l 1 I 240 200 I60 I20 80 4O 0 ENTHAL PV CHANGE DONALD L.$M/THJOHN L FERRELL 5 $1M eh:

ATTORNE V Dec. 19, 1967 D. L. SMITH ETAL NITROGEN LIQUEFACTION wrrnPLURAL worm 6 Sheets-Sheet 5 EXPANSION OF FEED AS REFRIGERANT Filed Oct.8, 1965 FIG. 4

TEMPERATURE -ENTHALPV PLOT FOR COLD EXCHANGER .35 ATM N/ 777065 NCOOLING CURVE V \Q'IQ RELATIVE POSI T/ONS T OF TURBINES 11v SECTIONCYCLE REHEA NO. I

REHEAT SECTION No.2

M m w M T m. m

OF TURBINE 5 IN C VCLE HEA T/NG CUR l/E [20 so ENT'HALPV CHANGE DONALDLSMITH Q 'NVENTORS JOHN L.FERRELL I ATTORNEY Dec. 19, 1967 D. L. SMITHETAL 3,358,460

NITROGEN LIQUEFACTION WITH PLURAL WORK EXPANSION 0F FEED AS REFRIGERANTFiled (Jot. 8, 1965 6 Sheets-Sheet 4.

FIG. 5

N/ TROGEN REFRIGERATION CVCLE WITH DOUBLE EXPANSION 8 REHEAT 4COMPRESSOR g? s 7 a b c REFRIGERATION l WARM UNIT 7 EXCHANGE? COMPRESSOR33 1 /62 COLD /EXCHANGER EXPANDER Y 4/ COMPRESSOR L IOU/D PRODUCT DONALDL. SMITH 3 5i JOHN L. FERRELL ATTORNEY Dec. 19, 1967 D. SMITH ETALNITROGEN LIQUEFACTION WITH PLURAL WORK EXPANSION OF FEED AS REFRIUERANT6 Sheets-Sheet 5 Filed Oct. 8, 1965 mm W DONALD L. SMITH JOHN L .FERRELLb wvbw QK WX A TTORNE V Dec. 19, 1967 D. L. SMITH ETAL 3,358,460

NITROGEN LIQUEFACTION WITH PLURAL WORK EXPANSION OF FEED AS REFRIGERANTFiled on. s, 1965 s Sheets-Sheet 6 2/6 LIQUID NITROGEN STORAGEREFR/GERAT/ON UN/T DONALD L SMITH JOHN L. FERRELL A NM-' ATTORNEY F IG.7

United States Patent Ofifice 3,358,469 Patented Dec. 19, 1967 NI'IRGGENLIQUEFACTION WITH PLURAL WORK EXPANSION OF FEED AS REFRIG- BRANT DonaldL. Smith, Berkeley Hei hts, and John L. Ferrell, North Plat-infield,N.J., assignors to Air Reduction Company, Incorporated, New York, N.Y.,a corporation of New York Filed Get. 8, 1%5, Ser. No. 494,177 11 Claims.(Cl. 62-9) This invention relates in general to the separation andliquefaction of low-boiling gases; and more particularly, to theliquefaction of nitrogen.

It is advantageous to operate liquefaction systems for low-boiling gasesat relatively low head-pressures, say, of the order of 600 to 1,160pounds per square inch absolute, since under such conditions an improvedproduct may be realized while effecting substantial reductions incapital and maintenance expenditures. For example, centrifugalcompression and expansion machinery, which is adapted for use in arelatively low pressure range, operates in such a manner that theproduct is substantially free of oil contamination. In addition, lessexpensive types of heat exchangers can be employed in the lower pressureranges.

However, many liquefaction systems employing headpressures within therange mentioned have the disadvantage of being thermodynamicallyinefiicient. A possible explanation for this is that in a systememploying a relatively high head-pressure, the high pressure stream inthe cold-le heat exchanger cools substantially as a straight-linefunction of temperature, whereas this is not true in a system employinga relatively low head-pressure. In the latter, the high-pressure streamundergoes a change in the cooling rate within the low temperature rangewhich causes an iriiection in the cooling curve, thereby producing asubstantial temperature spread with the warming curve of the lowtemperature returning stream. This discrepancy leads to the consumptionof substantial amounts of power, making such a system inefiicient.

Accordingly, it is a principal object of this invention to improve theefiiciency of liquefaction systems for lowboiling gases; and moreparticularly, to provide a liquefaction cycle for low-boiling gasesunder substantially reduced head-pressures which is characterized bygreater thermodynamic efficiency than heretofore achieved in thelow-pressure range. An ancillary object of the invention is to provide acycle for the liquefaction of low-boiling gases, more particularlynitrogen, which operates at increased eliiciency to produce a purerproduct, coupled with decreased maintenance and capital costs comparedto similar prior art cycles.

These and other objects are realized in accordance with the presentinvention in a cycle for the liquefaction of low-boiling gases, moreparticularly, nitrogen, which contemplates the use of head-pressureswithin the range 600 to 1,100 pounds per square inch absolute, wherein amajor portion of the pressurized stream is deployed to cool a minorportion thereof, without any substantial liquefaction occurring duringthis heat exchange. The salient feature of this arrangement is thework-expansion of the deployed major portion in two or more steps withone or more reheat steps intervening.

At head-pressures within the range, say, 600 to 1,100 pounds per squareinch absolute, an arrangement of the type mentioned performs thefunction of approximating the shape of the low-pressure warmingcharacteristic of the returning streams in the cold-leg heat exchangerto the inflected shape of the cooling characteristic of thehigh-pressure stream, thereby decreasing the temperature spread betweenthe characteristics and increasing the thermodynamic efiiciency of thesystem.

For the purposes of illustration, the invention is described withreference to a system for air separation and the liquefaction of oxygenand nitrogen, in which the stream of nitrogen gas from the separator isfirst compressed through seven stages, then partially cooled, afterwhich the initial stream is separated into two streams. The firststream, about two-thirds by volume, is workexpanded, then reheated, andagain work-expanded. This expanded component combines with returningcold vapor, and then passes back through heat exchangers countercurrentto the second stream which consists of the remaining third of theinitial stream. The second stream is cooled in the heat exchangerswithout liquefaction after which it passes through a throttle-valve forisenthalpic expansion, whereby a portion is converted to liquid which isseparated out from the gaseous phase in -a vapor-liquid separator. Thisliquid is again isenthalpically expanded to a pressure slightly aboveatmospheric pressure, at which pressure the liquid fraction is againseparated out and stored. The loW pressure vapor, including fiash vaporfrom the surface of the stored liquid, is returned through the heatexchangers in countercurrent with the high pressure streams from whichheat is absorbed, to the first compression stage for recycling throughthe system.

Whereas in the particular embodiment disclosed, the nitrogenliquefaction system includes two work-expansion steps separated by areheat step, it will be apparent to those skilled in the art thatimproved results might be obtained under certain circumstances by theinclusion of additional work expansion and reheat steps.

By way of further illustrating the present invention, a nitrogenliquefaction system is disclosed in combination with an air separationsystem, in such a manner that a portion of the liquid nitrogen productserves as a refrigerating reflux for one of the rectification columns ofthe air separation system, and also, as a refrigerant for condensingfiash vapor rising from the surface of the stored liquid oxygen.

Although the present invention is described herein with reference to anitrogen liquefaction system, it will be understood by those skilled inthe art that the principles disclosed are applicable to the liquefactionof any gas having a boiling point below about l12 E, including, forexample, oxygen, ethane, ethylene, methane, argon, fluorine, carbonmonoxide, neon, hydrogen and helium.

Other objects, features, and advantages of the system will be apparentfrom a detailed study of the specification hereinafter with reference tothe attached drawings, in which:

FIGURES l, 2, 3, and 4 are graphs illustrating the theory of the presentinvention, showing temperature in degrees Kelvin plotted againstenthalpy change for the cooling and heatin curves in the cold-legexchanger of a nitrogen liquefaction system. FIGURE 1 illustrates thecharacteristics of a prior art system having a head-pressure ofatmospheres; FIGURE 2 illustrates characteristics of a prior art systemutilizing a reduced headpressure of 45 atmospheres; FIGURE 3 illustratescharacteristics of an improved system in accordance with the presentinvention at 45 atmospheres head-pressure employing two worlr expansionsteps; and FIGURE 4 shows the characteristics of a projected system at35 atmospheres head-pressure employing three work expansion steps inaccordance with the present invention;

FIGURE 5 shows a nitrogen liquefaction cycle in accordance with thepresent invention, in a block diagram schematic; and

FIGURES 6 and 7 show an over-all detailed schematic for a combined airseparation and oxygen and nitrogen liquefaction system in accordancewith the present invention, the two figures adapted to be fittedtogether in the manner indicated in FIGURE 8.

An objective of this liquefaction cycle is to minimize the powerconsumption requirements. In order to achieve low power consumption, thetemperature difference between the countercurrent streams in the heatexchangers should be kept to a minimum.

In FIGURE 1 of the drawings, which shows temperature in degrees Kelvinplotted against change in enthalpy, in joules per gram-mol, for the coldexchanger in a nitrogen liquefaction system subject to 180 atmosphereshead-pressure, the cooling curve 1 for the high pressure stream and thewarming curve 2 for the low pressure return stream are both relativelystraight line characteristics of approximately the same slope. Becauseof this fact, the temperature difierence AT between the cooling andwarming streams remains substantially uniform throughout the exchanger,thus making a multi-step work expansion with intervening reheat of noparticular advantage in such a system.

Let us refer, now, to FIGURE 2 of the drawings. This shows a plot of thesame parameters for the cold exchanger in a nitrogen liquefaction systemsubject to a head pressure of 45 atmospheres. It is apparent that at thelower head-pressure the cooling curve 3 of the high pressure stream issubstantially inflected, producing a substantial spread, or temperaturedifference AT, with the warming curve 4 at the lower end of the scale.As explained before, this spread in AT makes the systemthermodynamically ineflicient, inasmuch as additional power must beconsumed to make up this difference.

In order to remedy this defect, and to provide a system which is morethermodynamically eflicient at the lower head-pressures, the presentinvention contemplates that the shape of the warming characteristic inthe coldleg heat exchanger should be approximated to the inflected shapeof the cooling curve. This is brought about by causing components of thewarming stream to undergo a plurality of work expansion steps withintervening reheating.

For example, FIGURE 3 of the drawings shows curves plotted for anitrogen liquefaction system similar to that represented in FIGURE 2,subject to 45 atmospheres head-pressure, but modified to include anauxiliary stream which undergoes two work expansion steps, in thepositions indicated by 7 and 8, and an intervening reheat step in thecold-leg heat exchanger running countercurrent with the cooling stream.In this plot, the cooling curve 5 is inflected, and is similar tocooling curve 3 of FIGURE 2. The warming curve in this modified systemcomprises sections 6a, 6b, and 60, whose slopes tend to conform at thedifferent levels to the slope of the cooling curve 5, thereby increasingthe thermodynamic efficiency of the system.

FIGURE 4 of the drawings shows a nitrogen liquefaction system subject toa still lower head-pressure, 35 atmospheres, in which the sameparameters are plotted, as in the foregoing figures. The'cooling curve 9at this low head-pressure sustains an even greater inflection. In orderto compensate for this in accordance with the present invention, threework expansion steps and two reheat sections are included as indicatedschematically by the expanders 11, 12, and 13, and arrows 14 and 15.This produces an inflected warming curve a, 10b, 10c, 10d, and 10e whichapproximates the shape of cooling curve 9.

It will be understood that this approach can be applied to the use ofeven a larger number of work expansion steps and intervening reheatsteps.

A nitrogen liquefaction system embodying the principles set forth in theforegoing paragraphs is shown in block diagram in FIGURE 5 of thedrawings.

A stream of make-up gas, comprising nitrogen, say, 99.99% pure, atsubstantially atmospheric pressure and temperature, is brought into thesystem through conduit 20,'which flows into junction 21. This stream iscompressed through a plurality of compression stages 22 to a pressureof, say, 435 pounds per square inch absolute. The stream then passes outthrough conduit 23 to a pair of centrifugal compression stages 24 and26, connected by conduit 25, Where it is further compressed to apressure of 670 pounds per square inch absolute, passing out throughconduit 27.

The highly compressed stream then passes into channel a of warm-leg heatexchanger 28 where it is cooled to about -40 F. It is then withdrawnfrom an intermediate point in the heat exchanger through conduit 29, andpassed through a refrigerator 31, where it is cooled, say, to -l0l F.,before passing back into channel a of heat exchanger 28 through conduit32.

The high pressure primary stream, cooled to about F., passes out of heatexchanger 28 to junction 33, where it is separated into two streams. Thefirst, or major stream, comprising about two-thirds by volume, passesthrough conduit 34 to a work expansion stage 35, where it is expanded toa pressure of about pounds per square inch absolute, at a temperature of236 F. The cooled, expanded major stream then passes out through conduit36 and into channel d of cold-leg heat exchanger 37, where it isreheated to, say, 2l3 F. It then passes out through conduit 38 to asecond work expansion stage 39, where it is expanded to a pressure of,say, 40 pounds per square inch absolute, and a temperature of 289 F.

This cooled, expanded medium-low pressure stream then flows intojunction 42, where, joined by a medium low pressure stream from conduit59, it flows through conduit 43 into channel b of heat exchanger 37, andthrough conduit 44 into channel b of'heat exchanger 28, where it absorbsheat from counterflowing high pressure cooling streams. It then leavesheat exchanger 28 at a temperature of about 85 F., and is returnedthrough conduit 45 to the second stage of compressor 22 forrecompression and recycling in the system.

Simultaneously, the remaining third of the high pressure stream flowinginto junction 33 flows out through channel a of heat exchanger 37, Whereit is cooled to a temperature of, say, --287 F. by counterflowingstreams. It flows out of cold exchanger 37 through conduit 47, and intothe throttle valve 48 where it is isenthalpically expanded to a pressureof, say, 40 pounds per square inch absolute, and cooled to a temperatureof, say, 303 F. The stream then passes into a first separator 51 throughconduit 49, where the liquid portion is separated out, and the vaporphase passes out of the top of the vessel through conduit 59 to join themedium-low pressure gas stream flowing into junction 42 for returnthrough the heat exchangers. V The liquid 52 from the bottom ofseparator 51 passes out through conduit 53 and through a secondisenthalpic expansion step in throttle valve 54, Where it is expanded toa pressure of, say, 19 pounds per square inch absolute at a temperatureof, say, 3l6 F.' ThC fluid from this second expansion step flows into asecond separator 55, in which the liquid 56 is collected in the bottom,from which the liquid product is withdrawn through outlet valve 57. Thelow pressure flash gas from this separator passes out through the top ofthe vessel through conduit 58, channel c of cold-leg heat exchanger 37,connecting conduit 62, channel 0 of heat exchanger 28, and conduit 63.The latter joins junction 21 at the inlet to the system, where the lowpressure stream, warmed to about 85 F., and at a pressure of 15 poundsper square inch absolute, joins the incoming make-up stream forrecompression and recycling through the system.

A combined air separation and liquefaction system will now be describedin greater detail, which includes a nitrogen liquefaction cyclesubstantially similar to that described with reference to FIGURE 5, withseveral slight modifications whereby a portion of liquid nitrogen streamserves as refrigerating reflux in the low pressure column of the airseparation system, and provides refrigeration for condensing flash vaporrising off of the liquid oxygen storage tank.

Referring now to FIGURES 6 and 7 of the drawings, which fit together inthe manner indicated by FIGURE 8, there is shown, in detailed schematic,a combined air separation, oxygen and nitrogen liquefaction system,illustrative of the principles of the present invention.

For the purposes of the present illustration, a separation-liquefactionsystem will be described which is designed to produce liquid oxygen atthe rate of 100 tons per day and liquid nitrogen'at the rate of 304 tonsper day.

Referring to FIGURE 6 which shows the air separation unit, air flowingat the rate of about 8,000 standard cubic feet per minute, a temperatureof 90 F., and at atmospheric pressure, is introduced through aconventional, non-lubricated type of fiberglas air filter 71 to thesuction inlet of a conventional oil-free compressor 72, Where the air iscompressed to about 110 pounds per square inch absolute. Compressor 72,in the present example, is a three-stage integral-gear centrifugalcompressor with full cooling.

After compression, the air stream passes through aftercooler 73, whereit is water cooled to a temperature of about 95 F., and thence throughwater separator 74, from which it flows through a refrigerator 76,consisting of a refrigeration circuit 75 which employs one ofthefluorocarbon refrigerants. This unit operates to bring the temperatureof the stream down to 40 F. The stream then passes through another waterseparator 77 to junction 78 where the stream passes through a pair ofconventional drying chambers 79 and 81, where it is dried to a very lowdew point.

The dried stream then passes at a pressure of 96 pounds per square inchabsolute through conduit 82 to the channel a of a heat exchanger 83where the temperature is reduced to l25 F. by counterflowing streams ofair and nitrogen. In the present example, this heat exchanger is aconventional type, consisting of brazed aluminum cores.

Carbon dioxide impurity and any remaining water vapor are removed fromthe stream as it passes from the junction 84 through the dual-bedabsorption chambers 85 and 86, adapted for low temperature removal ofcarbon dioxide by means of absorbing materials such as the zeolites,known in the art as molecular sieves. The unit is adapted to bereactivated every 16 hours using lowpressure cycle nitrogen. The airstream then Passes at a pressure of about 90 pounds per square inchabsolute through channel a of a second heat exchanger 3%, where thetemperature is reduced to 287 F At the aforesaid temperature and at apressure of about 89 pounds per square inch absolute, the stream entersinto the bottom of the high pressure rectification column 91, which inthe present illustration is a conventional type. The air is separated bydistillation in column 91 into a bottom liquid product containing about38% oxygen, which in the specification hereinafter will be referred toas rich liquid. Pure nitrogen gas at the top of column 91 passes outthrough the outlet pipe 113 and into the reboiler 106, Where it iscondensed by heat exchange with evaporating liquid oxygen from thebottom of low pressure column 103.

About 60% of the pure liquid nitrogen from reboiler 106 passes back intothe top of high pressure column 91 through inlet pipe 115, where it isused for reflux, while the remainder (3,400 standard cubic feet perminute) passes into coil 116 of the subcooler 95, which in the presentexample is a wound tube aluminum exchanger having three tube passes anda single shell pass. In subcooler 95 this liquid nitrogen is subcooledby cold lowpressure nitrogen gas passing in the outer shell, to atemperature of -305 F., at a pressure of pounds per square inchabsolute, before it passes to junction 118, under control of valve 120,and through inlet 119 into the top of low-pressure rectification column103, where it serves as reflux.

The rich liquid flows out of the bottom of the high pressurerectification column 91 through conduit 93 at the rate of 4,680 standardcubic feet per minute, at a temperature of -278 F., and a pressure of 89pounds per square inch absolute, through coil 94 of subcooler 95, andthrough the junction 96 into the silica gel absorber 97-98, wherehydrocarbon impurities, such as acetylene, are removed before it passesthrough valve 101 and through inlet 102 entering as a feed-stream intothe low pressure column 103, at a point part-way up in the column.

The final separation of oxygen and nitrogen occurs in the low pressurecolumn 103, where all of the air is separated into pure products exceptfor a small control stream. The latter serves to stabilize operation ofthe column, passing out through outlet pipe 133 at the rate of 321standard cubic feet per minute, a temperature of 307 F., and a pressureof 20 pounds per square inch absolute. This stream ultimately passesthrough heat exchanger column 89d, connecting pipe 134, and column d ofheat exchanger 83, to outlet 135, where it is vented to the atmosphereat about 36 F. i

As previously pointed out, reflux for low pressure column 103 issupplied by condensed liquid nitrogen from high pressure column iilthrough valve 120. This is augmented for refrigeration purposes bya'stream of liquid nitrogen flowing from the nitrogen liquefaction unitthrough conduit 122 under control of valve 123. In the present example,this stream flows at the rate of 2,066 standard cubic feet per minute,at a temperature of 303 F., and a pressure of 40 pounds per square inchabsolute. It will be apparent that valves 120 and 123 will regulate therespective flows to provide the desired degree of refrigeration in thereflux stream.

Pure liquid oxygen flows out of the bottom of the low pressure column103, through conduit 104 and into junction 105, where a portion passesthrough the column b of the reboiler 1136 where it is evaporated by heatexchange with condensing liquid nitrogen from the high pressurerectifying column 1, and passes back into low pressure column 103,through inlet 107 at the lower end.

Liquid oxygen product from the low pressure column 103, flowing at therate of 1,676 standard cubic feet per minute, at a temperature of 289F., and a pressure of 23 pounds per square inch absolute, is pumped outof junction by means of pump 169 through the coil 110 of subcooler 95,where it is reduced to a temperature of -299 F. at a pressure of 40pounds per square inch absolute, and ultimately through conduit 111 tothe liquid oxygen storage tank 112, shown in FEUURE 7.

Pure nitrogen from the top of low pressure column 103 passes out throughvent 124, at the rate of 8,159 standard cubic feet per minute, atemperature of 3l6 F. and pressure of 19 pounds per square inchabsolute. It fiows through the outer shell of subcooler $5, cooling theliquids passing through the inside coils, and then flows through conduit126 at a temperature of 2 83 F. to channel b of heat exchanger 89,connecting conduit 127 and channel b of heat exchanger 83, where thetemperature is raised to 36 F. at 15 pounds per square inch absolutepressure. From here it passes through conduit 1255 as the make-up streamfor the nitrogen liquefaction cycle, to be described presently Pureoxygen vapor passes through conduit 130 at the lower end of low pressurecolumn 103, at a temperature of 289 F. and a pressure of 23 pounds persquare inch absolute, through channel 0 in each of heat exchangers 89and 83, and is vented into the atmosphere through vent 132 An auxiliarypath is provided between the high pres- 7 sure column 91 and the lowpressure column 103,

through outlet 136 at the lower end of column 91 so that a portion ofthe cooled air stream, about 25% or less, coming into the lower end ofcolumn 91 can be withdrawn if desired under control of valve 137,partially rewarmed in channel 2 of heat exchanger 89, expanded with theproduction of work in expander 139, and introduced into low pressurecolumn 103 at a point below the middle, through inlet 142 under controlof valve 141. This auxiliary path provides means, if desired, for savingpower in the refiigeration system, at the expense of lower oxygenrecovery and lower purity by the nitrogen product.

Referring now to FIGURE 7, this shows a schematic diagram in accordancewith the present invention of a nitrogen liquefaction circuit to whichthe make-up stream of gas is furnished through conduit 128. In thepresent example, the following is a typical analysis of the impuritycontent of the nitrogen make-up stream:

In the example under description, which anticipates a liquid nitrogenoutput of 304 tons per day, the stream of nitrogen flows from conduit128 into the junction 129 at the rate of 8,159 standard cubic feet perminute, a temperature of 36 F., and a pressure of about 15 pounds persquare inch absolute.

The valve 149 can be utilized to control the rate of flow of gas fromthe junction 129. In the present example, gas flows at the rate 7,998standard cubic feet per minute from junction 129 intojuuction 150, at atemperature of 36 F., and a pressure of 15 pounds per square inchabsolute. The low pressure nitrogen stream returning through conduit 221flows into junction 150 at the rate of 637 standard cubic feet perminute, a temperature of 85 F., and a pressure of 15 pounds per squareinch absolute.

The low pressure stream flows out of junction 150 to the firstcompression stage 151 at the rate of 8,535 standard cubic feet perminute, at a temperature of 50 F., and pressure of 15 pounds per squareinch absolute, which is raised to approximately 32 pounds per squareinch absolute in the first stage. The stream from the first stage iscooled down in after-cooler 152, and flows into junction 201, where itis joined by the medium pressure return stream flowing through conduit199 at the rate 25,665 standard cubic feet per minute, at a temperatureapproximating 85 F., and pressure of 32 pounds per square inch absolute.

The fully compressed nitrogen then flows through the rate of 34,200standard cubic feet per minute through four additional stages ofcompression, 153, 155, 157, and 159, each followed by a respectiveafter-cooler 154, 156,

, 158, or'160. The stream emerges from the fifth compression stage andits after-cooler into conduit 162 at a pressure of 430 pounds per squareinch absolute and a temperature of 95 F.

The compressors 151, 153, 155, 157, and 159 are conventional types whichare operated in series fiom a common 10,000 horsepower motor, andcontrolled by means of suction-throttling and discharge pressurecontrol. Reverse rotation is prevented by a check valve installed in thedischarge line. The intercoolers and aftercoolers 152, 154, 156, 158,and 160 are conventional types of water coolers.

The partially compressed stream passes through conduit 162 to aseries-connected pair of centrifugal compressors, of conventional form,in which the stream is compressed in two steps to 670 pounds persquare'inch absolute, at a temperature of F. Each of these compressorsis followed by a conventional after-cooler, 164 and 167 respectively.Moreover, the centrifugal compressors are respectively coupled to a pairof expansion turbines 194 and 188, which are mounted on the same shaft,in such a manner that the compressor wheels serve as a brake for theturbines, absorbing the work of the expanding gas.

The fully compressed nitrogen then flows through the conduit 168 intochannel a of heat exchanger 169 for cooling by the outgoing low pressureand medium pressure nitrogen recycle streams. This heat exchanger is abrazed aluminum type comprising five cores in parallel flow.

At the point in channel a of exchanger 169 where the high pressurenitrogen stream reaches 40 F., the total stream is withdrawn from theheat exchanger 169 through conduit 170 and passed into a conventionalrefrigeration unit where it is cooled from -40 F. to 101 F.

In the present example, the refrigeration system 175 comprises twodifferent refrigeration cycles operating in cascade to cool the highpressure nitrogen stream. The

first of these refrigeration cycles'employs dichlorodifiuoromethane CCIF known by the trade-name Freon 12; and the second cycle employsmonochlorotrifluoromethane, known by the trade-name Freon 13. (Both ofthese are registered tradenames of E. I. du Pont de Nemours andCompany.) The nitrogen stream emerging at -40 F. from channel a of heatexchanger 169 flows through conduit 170 under a pressure of 669 poundsper square inch absolute, at the rate of 34,200 standard cubic feet perminute into the refrigeration unit 175.

After the cooled nitrogen stream hasreturned from refrigeration unit175' to channel a of heat exchanger 169 at a temperature of 101 F., itis further cooled by the counter-flowing fluids in channels b and 0thereof to -130 F. at which temperature and a pressure of 663 pounds persquare inch absolute it leaves the heat exchanger 169 and flows intojunction 186. At this point approximately 70% of the high pressurenitrogen stream (24,200 standard cubic feet per minute) is drawn offinto conduit 187. This major portion passes into turboexpander 188,where it is expanded with the production of work to a pressure of 147pounds per square inch absolute, and is thereby cooled to a temperatureof -236 F. As pointed out previously turboexpander 188, and itscompanion, turboexpander 194, used in the next stage, are of the radialinflow type. These turboexpanders are mounted on the same shaft with anddrive the respective centrifugal compressors 166 and 163; they arelubricated by a common system. Variable inlet nozzles control the flowthrough the turboexpanders which regulates the power available tocompress the cycled nitrogen.

The stream, expanded to 147 pounds per square inch absolute, passes outof turboexpander 188 through conduit 189 and is reheated in channel d ofheat exchanger 192 to a temperature of 213 F. The stream is then againexpanded with the performance of work through a second stage comprisingthe turboexpander 194 from a pressure of 145 to a pressure of 35 poundsper square inch absolute and is cooled to a temperature of 289 F. Themedium-low pressure stream passes out of turboexpander 194 throughconduit 195 and into junction 196, where it re-enters channel b of heatexchanger 192 through conduit 197, in combination with the returnmedium-low pressure flow through conduit 210. The combined streams passthrough conduit 197 into channel b of heat exchanger 192, and throughconduit 198 into channel [7 of heat exchanger 169, where they are heatedup to a temperature of 85 F. Flowing at the rate of 25,665 standardcubic feet per minute, the stream then passes through conduit 199 andinto the suction end of the second compression stage, through junction201, at a pressure of 32 pounds per square inch absolute for recyclingin the liquefaction system.

The approximately 30% of the high pressure stream remaining at junction186, where the major portion was diverted through the turboexpanders,passes through channel a of heat exchanger 192, where it is cooled to asaturated fluid of a single phase at about -269 F. This product stream,flowing at the rate of approximately 10,000 standard cubic feet perminute in conduit 203, and at a pressure of 660 pounds per square inchabsolute, is throttled (isenthalpically expanded) to 19 pounds persquare inch absolute in two steps. The stream is first expanded throughJoule-Thompson expansion valve 204, which is a conventional cryogenicthrottle-valve, to approximately 40 pounds per square inch absolute, ata temperature 303 F. This fluid passes into vessel 206 atop the liquidoxygen storage tank 112. Vessel 206, which is a conventional type ofliquid separator, serves as a liquid-vapor separator as Well as acondenser for flash oxygen vapor from the tank, which flows back intothe oxygen tank through receptacle 206a.

The nitrogen flash gas passing out of the top of separator 2G6, movesthrough conduit 210 to junction 196, flowing at the rate of 1,465standard cubic feet per minute, at a temperature of -303 F., and apressure of 36 pounds per square inch absolute. The combined lowpressure stream then passes through channel b of each of the exchangers192 and 169, joining the suction portion of the second compression stageat 85 F., as previously described.

The liquid fraction of nitrogen passes out of separator 206 throughconduit 207, and into junction 121, flowing at the rate of 8,535standard cubic feet per minute, at a temperature of 303 F., and pressureof 40 pounds per square inch absolute. Part of the liquid nitrogenserves as refrigerating reflux for the low pressure column 103 of theair separation system, passing through conduit 122 at the rate 2,066standard cubic feet per minute. The remainder (flowing at the rate of6,469 standard cubic feet per minute) passes through conduit 298 to asecond Joule-Thompson expansion valve 209, where it is again throttled(isenthalpically expanded) to a pressure of 19 pounds per square inchabsolute, and a temperature of 3l6 F. It flows into a second separator211 at the rate of 6,469 standard cubic feet per minute, where theliquid nitrogen separates out and is expanded into the storage tank 214at a pressure of 15 pounds per square inch absolute through valve 213.Both the liquid oxygen storage tank 112 and liquid nitrogen storage tank214, are conventional insulated types of a form well-known in the art.

Saturated nitrogen vapor flows out of separator 211 at the rate of 430standard cubic feet per minute, a temperature of --316 F., and apressure of 19 pounds per square inch absolute, through conduit 215 tojunction 218 where it is joined by flash nitrogen vapor from stor agetank 214, pumped through conduit 216 by a conventional cold blower pump217, at a rate of 207 standard cubic feet per minute, a temperature of303 F., and a pressure of 19 pounds per square inch absolute. Thecombined low pressure stream passes through conduit 219 and channel ofheat exchanger 192. It then passes through conduit 220 to channel c ofheat exchanger 169, where it emerges at a temperature of 85 F, flowingthrough conduit 227 into junction 150 at a rate of 637 standard cubicfeet per minute and at a pressure of 15 pounds per square inch absolute,through which it returns to the first compression stage for recycling inthe nitrogen liquefaction system.

It will be apparent to those skilled in the art that this invention isnot limited to the specific embodiments disclosed by way ofillustration; but rather, that the scope of the invention is defined interms of the appended claims.

We claim:

1. In a cycle for the liquefaction of low boiling gas which comprisesthe steps of:

compressing a primary stream including the feed stream of said gas to ahead pressure not exceeding about two and one-half times the criticalpressure of said cooling said compressed primary stream through a firstcooling step in a first heat exchanger,

further cooling at least a portion of said compressed primary streamthrough a second cooling step in a second heat exchanger, withoutsubstantial liquefaction thereof, isenthalpically expanding saidcompressed portion after said second cooling step to a low pressure toform a liquid fraction and a remaining vapor fraction, and

returning the remaining low pressure vapor fraction of said portion atsaid low pressure for recompression and recycling through a pathincluding said heat exchangers in countercurrent with streams beingcooled including said compressed portion;

the improvement which comprises:

separating said compressed primary stream before said second coolingstep into a major portion and a minor portion, whereby the compressedportion further cooled in said second heat exchanger is said minorportion,

withdrawing and expanding said major portion through a plurality of workexpansion steps wherein said major portion is cooled and expanded to amedium-low pressure without liquefaction,

each of said work expansion steps being separated by an interveningreheat step wherein the stream comprising said major portion is warmedup in said second heat exchanger in countercurrent with streams beingcooled including said compressed minor portion,

and after said final work expansion step, returning said major portionat a medium-low pressure without liquefaction through said heatexchangers in countercurrent with streams being cooled including saidcompressed minor portion, for recompression and recycling.

2. In a cycle for the liquefaction of low boiling gas in accordance withclaim 1, expanding said major portion through two work expansion stepsseparated by one intervening reheat step wherein the stream comprisingsaid major portion is warmed in countercurrent with said minor portionbeing cooled.

3. A cycle for the liquefaction of low boiling gas in accordance withclaim 1 wherein said step of compressing said primary stream of gastakes place in a plurality of stages, wherein said low pressure vapor"naction is returned to the initial compression stage for recompressionand recycling, and wherein said medium-low pressure major portion isreturned to a later compression stage for recompression and recycling.

4. In a cycle for the liquefaction of low boiling gas which comprisescompressing a primary stream of said gas to a head pressure notexceeding about two and one-half times the critical pressure of saidgas,

cooling said compressed primary stream through a first cooling step,further cooling at least a portion of said compressed primary streamthrough a second cooling step in heat exchanger means, wherein aninflected cooling characteristic is produced in said heat exchanger,

isenthalpically expanding said compressed and further cooled portion toa low pressure to form a liquid fraction and a remaining vapor fraction,

and returning the remaining low pressure vapor fraction of said portionfor recompression and recycling through a path including said heatexchanger means withdrawing and expanding said major portion through aplurality of work expansion steps, cooling and expanding said majorportion to a medium-low pressure Without liquefaction, each of said workexpansion steps being separated by an intervening reheat step whereinthe stream comprising said major portion is warmed up in said heatexchanger means in countercurrent with streams being cooled includingsaid compressed minor portion, and following said final work expansionstep, returning said major portion at medium-low pressure withoutliquefaction through said heat exchanger means in countercurrent withstreams being cooled including said minor portion for recompression andrecycling, thereby producing a composite warming characteristic whichapproximates the inflected shape of the cooling characteristic of thestream comprising said compressed portion. 5. A cycle for liquefyingnitrogen gas which comprises the steps of compressing a primary streamof nitrogen including a feed stream through a plurality of compressionstages to an elevated pressure within the range 40 to 75 atmospheres,cooling said compressed primary stream by means including a first heatexchanger to a temperature within the range -l20 F. to 130 F.,subsequently separating said compressed primary stream into a majorstream consisting of between 65 and 75% by volume of said primarystream, and a minor stream consisting of the remainder, further coolingsaid compressed minor stream through a second heat exchanger,isenthalpically expanding said compressed minor stream to a low pressureto form a liquid fraction and a remaining vapor traction, returning theremaining low pressure vapor fraction through said heat exchangers incountercurrent with streams being cooled including said compressed minorstream for recompression and recycling, expanding said compressed majorstream through a plurality of work expansion steps wherein said majorstream is cooled and expanded to a medium-low pressure Withoutliquefaction, each of said work expansion steps being separated by areheat step wherein said major stream is warmed up in said second heatexchanger in countercurrent with said cooling compressed minor stream,and after said final work expansion step returning said major stream ata medium-low pressure without liquefaction for recompression andrecycling through a path including said heat exchangers incountercurrent with streams being cooled including said compressed minorstream. 6. A cycle for the liquefaction of nitrogen in accordance withclaim wherein said major stream is expanded through a first workexpansion step to a pressure less than a quarter of said elevatedpressure and a temperature within the range 235 F. to 245 F., said majorstream is subsequently reheated in said second heat exchanger incounter-current with said compressed minor portion to a temperaturewithin the range -210 F. to 220 F., and said major stream is finallyexpanded in a second work expansion, step to a pressure slightly inexcess of 2.5 atmospheres and cooled to a temperature within the range-285 F. to 305 F.,

prior to returning without liquefaction through said heat exchangers. 7.In a cycle for the liquefaction of nitrogen in accordance with claim 5wherein said step of compressing said primary stream takes place in aplurality of stages, wherein said low pressure vapor fraction isreturned to the initial compression stage for recompression andrecycling, and wherein said medium-low pressure major portion isreturned to a later compression stage for recompression and recycling.

8. A cycle for the liquefaction of nitrogen in accordance with claim 5wherein said compressed primary stream is initially cooled in said firstheat exchanger to a temperature within the range -35 F. to 45 F.,

said compressed cooled primary stream is then withdrawn laterally fromsaid heat exchanger and cooled by external refrigeration means to atemperature within the range F. to F.,

and said further cooled primary stream is again passed into said firstheat exchanger where it is cooled to a temperature within the range l20F. to F. by heat exchange with counterflowing streams in said heatexchanger.

9. In an air separation and liquefaction cycle which comprises incombination the steps of:

removing impurities from a feed-stream of said air including carbondioxide and water,

compressing said feed-stream to an elevated pressure,

cooling said feed-stream to approximately its satura tion temperature byheat exchange with outgoing gas streams, separating said feed-stream bydistillation in a high pressure rectification column into a bottomproduct called rich liquid containing substantially more than 20% byvolume of oxygen and an overhead high purity nitrogen product,

subcooling and further separating said rich liquid by distillation in alow pressure rectification column into a high purity liquid oxygenproduct and an overhead high purity nitrogen product,

condensing the nitrogen product of said high pressure rectificationcolumn by reboiling said liquid oxygen 7 product from the bottom of saidlow pressure rectification column,

utilizing a major portion of said condensed liquid nitrogen product asreflux for said high pressure rectification column,

subcooling and utilizing the remainder of said condensed liquid nitrogenproduct for reflux in said low pressure rectification column,

storing the liquid oxygen product of said low pressure rectificationcolumn in an insulated storage vessel,

passing a stream of high purity nitrogen gas overhead from said lowpressure rectification column through subcooling means forconcurrent'streams of rich liquid oxygen and liquid nitrogen,

utilizing said stream of high purity nitrogen as a feedstream for aliquefaction cycle operating in conjunction with said air separationcycle,

compressing and cooling nitrogen feed-stream through a first coolingstep in a first heat exchanger,

further cooling at least a portion of said compressed nitrogenfeed-stream through a second cooling step in a second heat exchangerwithout substantial lique: faction thereof,

isenthalpieally expanding said compressed nitrogen portion after saidsecond cooling step to a low pressure to form a liquid fraction and aremaining vapor fraction, and

separating and returning the remaining low pressure vapor fraction ofsaid portion at said low pressure for recompression and recyclingthrough a path in:

cluding said heat exchangers in countercurrent with streams being cooledincluding the compressed nitrogen feed-stream;

the improvement which comprises:

separating said compressed nitrogen feed-stream before said secondcooling step in said nitrogen liquefaction cycle into a major portionand a minor portion whereby the compressed portion further cooled insaid heat exchanger is said minor portion,

withdrawing and expanding said major portion through a plurality of workexpansion steps wherein said major portion is cooled and expanded to amedium-low pressure without liquefaction,

each of said work expansion steps being separated by an interveningreheat step, wherein the stream comprising said major portion is warmedup in said second heat exchanger in countercurrent with streams beingcooled including said compressed minor portion,

after said final work expansion step returning said major portion at amedium-low pressure without liquefaction through said heat exchangers incountercurrent with streams being cooled including said minor portionfor recompression and recycling,

utilizing a heat exchange between oxygen vapor rising off of saidinsulated storage vessel in said air separation system and saidisenthalpically expanded nitrogen portion for separating liquid andvapor in said portion and for recondensing said oxygen vapor to liquid,

and utilizing at least :a portion of the liquid fraction of saidnitrogen to supply refrigerating reflux to said low pressurerectification column in said air separation cycle.

10. An air separation cycle in combination with oxygen and nitrogenliquefaction cycles which comprises the steps of:

compressing a feed-stream of said air,

cooling said compressed stream of air through a path including heatexchanger means,

passing said cooled compressed stream of air into a high pressurerectification column for separation into rich liquid in the bottom ofsaid column comprising substantially more than 20% by volume of oxygen,and nitrogen gas in the top of said high pressure column,

further cooling said rich liquid derived from the bottom of said highpressure rectification column in a subcooler in a heat exchange withcounterflowing cold nitrogen vapor,

introducing said cooled rich liquid part-way up in a second low pressurerectification column,

passing the nitrogen vapor from the top of said high pressurerectification column through a condenser for condensation in a heatexchange with counterfiowing liquid oxygen, returning one portion ofsaid condensed liquid nitrogen to the top of said high pressure columnfor refluxing,

and passing another portion of said liquid nitrogen through saidsubcooler in countercurrent with cooled nitrogen vapor and into the topof said low pressure rectification column for refluxing,

passing purified liquid oxygen out through the bottom of said lowpressure column,

vaporizing part of said liquid oxygen in said condenser in a heatexchange with cold nitrogen vapor from the top of said high pressurerectification column and passing said oxygen vapor into said lowpressure column,

pumping part of said liquid oxygen through said subcooler for furthercooling and into an insulated receptacle for storage,

returning a control stream of cold low pressure nitrogen from near thetop of said low pressure rectification column through said heatexchanger means in countercurrent with high pressure cooling streams,and ultimately releasing said stream to the atmosphere,

returning a stream of cold oxygen vapor from near the bottom of said lowpressure rectification column through said heat exchanger means andreleasing said stream into the atmosphere,

returning a stream of purified cold nitrogen from the top of said lowpressure rectification column, and through said subcooler and heatexchanger means to said nitrogen liquefaction cycle as the make-upstream therefor,

in said nitrogen liquefaction cycle compressing said nitrogen streamthrough a plurality of compression stages terminating in at least onecentrifugal compressor stage,

passing said compressed nitrogen make-up stream into a first heatexchanger for cooling to a temperature within the range 35" F. to 40 F.,

withdrawing said compressed nitrogen stream laterally from said firstheat exchanger and passing it through refrigeration means comprising atleast one external refrigerating cycle, wherein said nitrogen make-upstream is cooled to a temperature within the range F. to F.,

again passing said compressed cooled nitrogen stream into said firstheat exchanger for cooling to a temperature within the range F. to F.,

separating said compressed nitrogen stream into a major streamconsisting of 65% to 75% by volume of the flow of said compressednitrogen stream, and a minor stream consisting of the remainder,

withdrawing said major compressed nitrogen stream through a firstexpansion turbine wherein said stream is work-expanded to a pressurewithin the range to pounds per square inch absolute and cooled to atemperature within the range 235 F. to 245 F.,

reheating said major stream to a temperature within the range -2l0 F. to220 F. in a second heat exchanger in countercurrent with said compressedminor stream,

re-expanding said major stream through a second expansion turbinewherein said stream is workexpanded to a medium-low pressure within therange 30 to 40 pounds per square inch absolute, and cooled to atemperature within the range 285 F. to 305 F.,

returning said medium-low pressure stream through said heat exchangersto a stage following the initial stage of said compression forrecompression and recycling,

passing said minor compressed nitrogen stream through said second heatexchanger in countercurrent with said low pressure returning majorstream and thereby cooling said minor stream to a temperature within therange 260 F. to 270" F. at a pressure within the range 650 to 11,000;

expanding said cooled compressed minor stream in a first isenthalpicexpansion step through a throttle valve to a pressure slightly above twoatmospheres, thereby to convert a fraction of said stream to liquid,

separating out the nitrogen liquid fraction from the nitrogen vaporfraction in a heat exchange with flash oxygen vapor in condensing meanscommunicating with said insulated receptacle for storing liquid oxygenfrom said air separation system,

eturning the medium-low pressure vapor fraction from said firstisenthalpic expansion step through said heat exchanger, together withthe medium-low ressure stream from said expansion turbine, to a stagefollowing the initial stage of said compressor for recompression andrecycling,

making a portion of said nitrogen liquid fraction available for use as areflux in the top of the low-pressure rectification column in said airseparation system,

passing the remainder of the nitrogen liquid fraction resulting fromsaid first isenthalpic expansion step into a valve for a secondisenthalpic expansion step to a pressure slightly above atmosphericpressure and a temperature below the liquefaction temperature ofnitrogen,

separating off the low pressure saturated nitrogen vapor fraction fromthe final liquid nitrogen fraction resulting from said secondisenthalpic expansion step inanother separating means, which includesflash nitrogen vapor from liquid nitrogen stored in said cycle,

passing said low pressure nitrogen vapor through said heat exchanger incountercurrent with cooling high pressure streams to the initialcompression stage of said nitrogen liquefaction cycle for recompressionand recycling,

References Cited UNITED STATES PATENTS.

Dennis 629 Koehn et a1. 629 Shaievitz et al 62--29 X Smith 6213 X Geistet al; 6229 Dennis v 6239 X NORMAN YUDKOFF, Primary Examiner.

20 W. PRETKA, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,358,460 December 19, 1967 Donald L. Smith et 8.1.

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

Column 7 lines 56 and 57 for "The fully compressed nitrogen then flowsthrough the rate of" read The augmented stream flows out of junction 201at the rate of column 15, line 2, for "as a reflux" read 3 reflux Signedand sealed this 10th day of June 1969.

(SEAL) Attest:

Edward M. Fletcher, Jr. WI LL I AM E Attesting Officer Commissioner ofPatents SCHUYLER, JR.

1. IN A CYCLE FOR THE LIQUEFACTION OF LOW BOILING GAS WHICH COMPRISESTHE STEPS OF: COMPRESSING A PRIMARY STREAM INCLUDING THE FEED STREAM OFSAID GAS TO A HEAD PRESSURE NOT EXCEEDING ABOUT TWO AND ONE-HALF TIMESTHE CRITICAL PRESSURE OF SAID GAS, COOLING SAID COMPRESSED PRIMARYSTREAM THROUGH A FIRST COOLING STEP IN A FIRST HEAT EXCHANGER, FURTHERCOOLING AT LEAST A PORTION OF SAID COMPRESSED PRIMARY STREAM THROUGH ASECOND COOLING STEP IN A SECOND HEAT EACHANGER, WITHOUT SUBSTANTIALLIQUEFACTION THEREOF, ISENTHALPICALLY EXPANDING SAID COMPRESSED PORTIONAFTER SAID SECOND COOLING STEP TO A LOW PRESSURE TO FORM A LIQUIDFRACTION AND A REMAINING VAPOR FRACTION, AND RETURNING THE REMAINING LOWPRESSURE VAPOR FRACTION OF SAID PORTION AT SAID LOW PRESSURE FORRECOMPRESSION AND RECYCLING THROUGH A PATH INCLUDING SAID HEATEXCHANGERS IN COUNTERCURRENT WITH STREAMS BEING COOLED INCLUDING SAIDCOMPRESSED PORTION; THE IMPROVEMENT WHICH COMPRISES: SEPARATING SAIDCOMPRESSED PRIMARY STREAM BEFORE SAID SEOND COOLING STEP INTO A MAJORPORTION AND A MINOR PORTION, WHEREBY THE COMPRESSED PORTION FURTHERCOOLED IN SAID SECOND HEAT EXCHANGER IS SAID MINOR PORTION, WITHDRAWINGAND EXPANDING SAID MAJOR PORTION THROUGH A PLURALITY OF WORK EXPANSIONSTEPS WHEREIN SAID MAJOR PORTION IS COOLED AND EXPANDED TO A MEDIUM-LOWPRESSURE WITHOUT LIQUEFACTION, EACH OF SAID WORK EXPANSION STEPS BEINGSEPARATED BY AN INTERVENING REHEAT STEP WHEREIN THE STREAM COMPRISINGSAID MAJOR PORTION IS WARMED UP IN SAID SECOND HEAT EXCHANGER INCOUNTERCURRENT WITH STREAMS BEING COOLED INCLUDING SAID COMPRESSED MINORPORTION, AND AFTER SAID FINAL WORK EXPANSION STEP, RETURNING SAID MAJORPORTION AT A MEDIUM-LOW PRESSURE WITHOUT LIQUEFACTION THROUGH SAID HEATEXCHANGERS IN COUNTERCURRENT WITH STREAMS BEING COOLED INCLUDING SAIDCOMPRESSED MINOR PORTION, FOR RECOMPRESSION AND RECYCLING.